Optical fibers are often bundled together in parallel fashion to form a product known as an optical fiber ribbon. The ribbon includes optical fibers that have been encased in a polymeric matrix material to secure the fibers in the parallel arrangement. The matrix portion of the ribbon can include one or more layers of the polymeric matrix material, and each optical fiber typically contains a dual layer coating system that includes a soft, inner polymer coating and a hard, protective outer polymer coating. Prior to forming the ribbon, the optical fibers may also be coated with a thin colored layer of marking ink (i.e., in a polymer base) for purposes of fiber identification within the ribbon.
The most basic function of the ink is to provide a means for identifying individual fibers in both ribbon and loose tube cables during installation. Photoinitiators are used to initiate the polymerization process when the inks are exposed to UV light during the inking process. The pigments also absorb light so obtaining a fast cure speed is a challenge. Fast cure speed is desired as the ink should be well cured prior to putting the fibers in a ribbon or cable. If the ink is under-cured this can cause problems including, but not limited to, increased surface friction, poor ribbon peel performance (where the matrix material forms a very high bond with the ink due to residual acrylate groups on the ink surface), and the propensity for the ink layer to come off of the fiber during ribbon handling or stripping. A fast cure speed is also desirable to ensure robust process performance. For example, if the lamp intensity becomes low or the quartz tube in the lamp assembly becomes dirty, less UV light will be available to cure the inks and the result will be a low degree of cure on the inked fiber. Inks with low cure speeds are also disadvantaged from the standpoint that inking line speeds cannot be increased to facilitate higher output. Therefore, there is a need for radiation curable marking inks that have sufficient cure speed to overcome the above described challenges.
During use, the ribbon unit must be stripped prior to splicing operations in the field. Stripping is usually performed using thermal strippers (e.g., Sumitomo ribbon stripper model JR-4A) at operating temperatures in the range of 70-100° C. For successful splicing, it is imperative that the polymer layers (inner and outer coatings, ink coating, matrix material) be removed from the ribbon cleanly and in an intact unit, i.e., leaving little debris on the stripped glass fiber. With an undesirable amount of debris, it is necessary to remove the debris, for example, by wiping the stripped fibers with an alcohol-moistened cloth. Unfortunately, with more debris, it is often necessary to wipe the fibers more than once. Correcting the problem of excessive debris therefore requires additional labor, time, and cost. Additionally, the act of repeated wiping may have the undesired consequence of weakening the glass fiber within the splice junction. For these reasons, wiping of the stripped glass fibers should be kept to a minimum.
Strip cleanliness is conventionally measured on a five-point scale, with a score of five being unclean and a score of one being clean. While the strip cleanliness will vary according to the needs, it is generally desirable for a stripped ribbon to possess optic fibers rated at a cleanliness of three or lower, more preferably two or lower. Tube-off is also measured on a five-point scale and is used as a means to assess the integrity of the polymers layers that are removed from the ribbon. A score of one means the polymer layers can be removed in an intact unit. A score of five means that there is total disintegration of the polymer layers and that they are not removed in an intact unit. As with the strip cleanliness, it is generally desirable for the optical fiber ribbon to possess a tube-off rating of three or lower, more preferably two or lower as this results in a reduced need to clean the strip tool, thus reducing splicing time.
Previous attempts to solve this problem have focused exclusively on the properties of the matrix material. One such approach is disclosed in U.S. Pat. No. 6,501,890 to Wilson et al., which suggests using a matrix material that exhibits a maximum tensile strength at 100° C. of at least about 1000 psi and an elongation at break at 100° C. of at least about 15 percent. Focusing on the properties of the matrix material, alone, ignores any interactions between the other polymeric materials that surround the glass fibers.
The present invention is directed to overcoming the above-noted deficiencies in the art, and achieving an optical fiber ribbon that possesses improved strip cleanliness and tube-off over a range of temperatures and strip conditions.